In AV apparatuses in recent years, a switching power supply circuit is mounted in many cases for reductions in power consumption and size, for example. It is known that the switching power supply circuit generates switching noises at comparatively high frequencies. In addition, in AV digital apparatuses represented, for example, by CD players, high-frequency noises are generated from digital circuits. In short, in the AV apparatuses and the like in recent years, there is a tendency of increases in high-frequency noises, as the so-called in-apparatus noises.
Those AV apparatuses which incorporate a radio tuner have been wide spread. Where such an AV apparatus having a radio tuner is generating the above-mentioned in-apparatus noises, the antenna for receiving the radio broadcasting wave meets the inconvenience that the noises are received as disturbance noises.
Besides, in recent years, electronic apparatuses have been digitized, so that the noises propagating through electric light wires tend to increase, and the noises from the light wires are also a major cause of the disturbance noises received by the antenna.
FIG. 6 schematically illustrates the principle of the above-mentioned reception of disturbance noises by an antenna.
An AV apparatus 20 is, for example, an apparatus including at least a radio tuner, and an antenna 30 is connected to the AV apparatus 20 through a feeder 31.
In the AV apparatus 20, the noise generated as above generates a noise potential between the AV apparatus 20 and ground. Here, for example, if the noise generated in the AV apparatus 20 is conducted through the feeder 31, a component as noise current flows into the feeder 31 and the antenna 30 when an electric wave is radiated from the antenna 30 due to the potential difference between the AV apparatus 20 and ground. As a result, the noise current is received by the antenna 30 as a disturbance noise.
A general example of the AM antenna in recent years is a loop antenna obtained by a method in which an about 1 m lead wire having a non-shield structure is formed into a loop shape. Therefore, where the antenna 30 shown in FIG. 6 is an AM antenna, the antenna tends to receive disturbance noises, which is particularly a problem.
In view of this, a configuration for making a countermeasure against noise in relation to a loop antenna, for example, is disclosed in Japanese Patent Laid-open No. Sho 57-2102. In the configuration described in Japanese Patent Laid-open No. Sho 57-2102, a coaxial cable composed of a core wire and a shield conductor surrounding the core wire is used for the loop antenna. In addition, the shield conductor of the coaxial cable is cut at positions equally spaced from input/output terminals. Incidentally, in the case of this configuration, the shield conductor in one coaxial cable is divided into two portions, with the cutting position as a boundary, and, therefore, these shield conductors are connected to the ground potential. This makes it possible to effectively reduce the noise received by the loop antenna, as compared with the case where the shield is applied to the whole part of the loop antenna, for example.
However, it is desirable for the loop antenna to be further improved in the reception of disturbance noises. It is an object of the present invention to further reduce the disturbance noise received by a loop antenna. It is another object of the present invention to ensure that the loop antenna provided with such a noise-reducing configuration can be manufactured efficiently.